Very low systemic perfusion (flow of blood) is caused by potentially reversible hemorrhage, sepsis (spread of bacteria) and cardiac arrest. When there is very low blood flow, generally due to low aortic pressure, the body directs a greater percentage of the flow to the brain and other organs that require continuous blood flow to survive (although the flow decreases even to such critical organs), while greatly decreasing the flow to other organs and tissues which can survive for a longer time without large blood flow. When perfusion to these less critical organs and tissues such as the stomach decreases, carbon dioxide resulting from metabolism is not rapidly carried away and the partial pressure of carbon dioxide increases. The measurement of CO.sub.2 and changes in pH resulting therefrom, are commonly made in the stomach and intestines to determine the extent of perfusion failure in a patient and to determine the effectiveness of treatment.
The measurement of CO.sub.2 is commonly made by threading a catheter through the nasal passage and the esophagus to the stomach, and sometimes through the stomach into the intestines, with a catheter sometimes being threaded through the anus into the colon. These procedures are invasive and can harm the patient.
In U.S. Pat. No. 5,579,763, applicants describe the introduction of a catheter with a carbon dioxide sensor through the nasal or oral passage into the esophagus. That patent presents the results of tests showing that measurements of CO.sub.2 in the esophagus are closely correlated with aortic pressure and they are even better correlated to aortic pressure than measurements of CO.sub.2 in the stomach. While CO.sub.2 measurements in the esophagus involve only moderate invasiveness, it is still invasive to the body since it involves moving a catheter down past the epiglottis to reach the esophagus. It would be desirable if an even less invasive method were available to measure perfusion failure and to indicate the state of the patient as a result of perfusion failure and as the result of blood infusion or other methods taken to increase perfusion.